[Interest] Qt LGPLv3 and Qt LGPL Exception

[Benjamin TERRIER]

Hi everyone,

Within the topic "[Development] Scope of source code license files" a
question came about Qt LGPLv3 licensing which didn't get an answer and
I would really like to get an answer from someone at Digia or The Qt
Company.


When Qt was licensed under the LGPLv2, Qt came with "The Qt Company Qt
LGPL Exception version 1.1" which allows to include Qt headers in
proprietary software. Without this exception Qt would not have been
usable to develop proprietary software when licensed under LGPLv2.
The technical reason is that when including Qt headers in proprietary
software, your final binary contains compiled forms of Qt code (e.g
inline function or template classes).

Starting from Qt 5.7, LGPLv2 in no more available and has been
replaced by LGPLv3. "The Qt Company Qt LGPL Exception version 1.1"
does not apply anymore as it is an exception to LGPLv2.
The LGPLv3 has by itself an exception to allow includes in proprietary
software, but there is an arbitrary limit that the exception does not
apply to functions of more than 10 lines.

The direct consequence is that Qt (licensed under LGPLv3)  is not
usable to develop proprietary software as developers will have to
ensure that every Qt header does not have any 10+ line long inline or
template function.

However, Qt website states that it is "Possible to keep your
application private with dynamic linking" when using LGPLv3
(https://www.qt.io/licensing-comparison/).

So is there supposed to be an LGPLv3 exception? If there is one and I
missed it where is it? Or did the Qt Company forget to write one?

Thanks,

BR

Benjamin Terrier